System and method for operating an electric motor by limiting performance

ABSTRACT

A method and system for limiting motor performance in a hybrid electric vehicle system. During a condition in a primary drivetrain, the method limits performance of an electric motor used in an auxiliary drivetrain to control energy consumed from a battery in the auxiliary drivetrain. A calculation or measurement is made to determine available battery energy remaining in the battery after the condition. The performance of the electric motor is then limited based on the available battery energy.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to electrically driven vehicles. Inparticular, the present invention relates to limiting motor performancein accordance with certain operating conditions of the vehicle.

2. Background Art

The present invention relates to electrically driven vehicles having“electric only” capabilities. Common “electric only” capable hybridsinclude a series hybrid electric vehicle (SHEV), a parallel hybridelectric vehicle (PHEV), and a parallel/series hybrid electric vehicle(PSHEV).

“Electric only” capable vehicles include at least two power sources,where one of the at least two power sources stores energy and one of thepower sources generates energy. The “electric only” designationindicates the vehicle can be driven with energy from the energy storingpower source (battery) if the energy generating power source (engine orfuel cell) is turned off or not generating power.

The energy storing power sources can be distinguished from the energygenerating power source because the storing power source must receiveenergy, rather than generating its own energy. Common energy storingpower sources are batteries and common energy generating power sourcesare engines and fuel cells which consume fuel and produce chemicalreactions to generate the electric energy.

Each of the power sources can be used to provide torque to wheels fordriving the vehicle. The software, electronics, and mechanism whichpermit the power sources to provide torque to the wheels are referred toas a drivetrain.

The drivetrain for the energy storing power source is referred to as anauxiliary drivetrain to distinguish it from the drivetrain for theenergy generating power source which is referred to as a primarydrivetrain. In this manner, the primary drivetrain includes thegenerating power source and the auxiliary drivetrain includes thestoring power source.

A problem may arise if the primary drivetrain experiences a conditionwhich limits or prevents it from providing torque to the wheels orproviding power to the auxiliary drivetrain. Assuming that the hybridvehicle only includes one primary drivetrain and the storing energysource in the auxiliary drivetrain is a battery, the continued drivingof the vehicle may be limited to the amount of available battery energyremaining in the battery.

The continued driving of the vehicle then becomes dependent on theremaining battery energy and how the remaining energy is used by anelectric driving motor used to drive the vehicle. Generally, it isdesirable to control the use of the remaining battery energy to prolongvehicle driving. Accordingly, there is a need for a method of optimallycontrolling the HEV when the primary drivetrain is limited or unable toprovide nominal performance.

SUMMARY OF INVENTION

The present invention meets the need identified above with a method tocontrol performance of a hybrid electric vehicle (HEV). In particular,the method limits performance of the HEV to prolong operation of theHEV.

One aspect of the present invention relates to a method of controlling aHEV having a primary drivetrain and an auxiliary drivetrain. The methodlimits performance of an electric motor used in the auxiliary drivetrainto control energy consumed from a battery in the auxiliary drivetrain.

The performance is limited according to performance limiting strategieswhich are based in part on the future availability or unavailability ofthe primary drivetrain to produce energy for storage in the auxiliarydrivetrain. The performance is limited further when the primarydrivetrain is unavailable in the future. This is done to prolongoperation of the vehicle by causing the electric motor to consume lessenergy than it would otherwise consume if the primary drivetrain wereavailable.

One aspect of the present invention controls the limiting of theelectric motor based on available battery energy remaining in thebattery after the condition. A calculation or measurement is made todetermine available battery energy such that the performance of theelectric motor is continuously limited based on the available batteryenergy.

The electric motor performance can be limited by setting a maximumvehicle speed limit based on the available battery energy. Preferably,the maximum vehicle speed is 45 mph if the battery state of charge isabove 50%, 35 mph if the battery state of charge is between 50% and 45%,25 mph if the battery state of charge is between 45% and 35%, and 15 mphif the battery state of charge is between 35% and 20%.

The electric motor performance can also be limited by setting a maximumpower limit for the electric motor based on the available batteryenergy. Preferably, the maximum power limit is set to 25 kW if thebattery state of charge is above 50%, 15 kW if the battery state ofcharge is between 50% and 45%, 10 kW if the battery state of charge isbetween 45% and 25%, and 5 kW if the battery state of charge is between35% and 20%.

The electric motor performance can also be limited by setting a maximumpower limit in combination with a maximum speed limit. Preferably, thecombined limits are set to 45 mph and 25 kW if the battery state ofcharge is above 50%, 35 mph and 15 kW if the battery state of charge isbetween 50% and 45%, 25 mph and 10 kW if the battery state of charge isbetween 45% and 35%, and 15 mph and 5 kW if the battery state of chargeis between 35% and 20%.

The electric motor performance can still further be limited by limitingactual power provided by the electric motor based on a relationshipbetween a maximum power limit and a maximum speed limit for the electricmotor. Preferably, the actual power is limited according to thefollowing algorithm:

${AP} = {{2*{MP}*\left( \frac{{MS} - {VS}}{MS} \right)} - {{MP}*\left( \frac{{MS} - {VS}}{MS} \right)^{2}}}$wherein:

AP=actual power (kW);

MP=maximum power (kW);

MS=maximum vehicle speed (mph); and

VS=actual vehicle speed (mph).

In addition to limiting a driving characteristic of the electric motorperformance, the limiting can comprise shutting down the vehicle if theavailable battery energy becomes so low that the primary drivetrain maynot be restarted. This may only be applicable to electric start hybridshaving engines which require starting torque form the energy storingdevice. Preferably, the vehicle is shutdown if the battery state ofcharge drops below 20%.

One aspect of the present invention relates to a HEV system which canlimit performance of the HEV. The HEV system includes an internalcombustion engine, a planetary gear set connected to the internalcombustion engine, and a number of meshing gears connected to theplanetary gear set to receive torque from the planetary gear set. A pairof wheels connect to the meshing gears to drive the vehicle in responseto received torque.

The HEV system further includes a generator connected to the planetarygear set, a battery connected to the generator for storing energy, andan electric motor connected to the battery to consume energy from thebattery and to provide torque to the differential gears.

To limit performance, the HEV system includes a vehicle systemcontroller. The controller controls consumption of available batteryenergy by the electric motor during a condition which prevents theengine from providing torque to the wheels and power to the battery.This is done by limiting a driving characteristic of the electric motorbased on the available battery energy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an exemplary hybrid electric vehicle system forlimiting motor performance during a condition in a primary drivetrain;

FIG. 2 illustrates power and torque flow in the hybrid electric vehiclesystem;

FIG. 3 illustrates a positive parallel/series mode of operation for thehybrid electric vehicle system;

FIG. 4 illustrates a negative parallel/series mode of operation for thehybrid electric vehicle system;

FIG. 5 illustrates a parallel mode of operation of the hybrid electricvehicle system;

FIG. 6 illustrates an electric mode of operation of the hybrid electricvehicle system;

FIG. 7 illustrates limiting motor performance by setting a maximum speedas a function of a battery state of charge;

FIG. 8 illustrates limiting motor performance by setting a maximum poweroutput as a function of a battery state of charge;

FIG. 9 illustrates limiting motor performance by setting a maximum poweroutput and a maximum speed as a function of a battery state of charge;and

FIG. 10 illustrates dynamically limiting motor performance bycontrolling actual power output as a function of vehicle speed, maximumpower, and maximum speed.

DETAILED DESCRIPTION

The present invention relates to electrically driven vehicles having“electric only” capabilities. “Electric only” capabilities refer tovehicles which can operate with an “electric only” architecture. Common“electric only” capable hybrids include a series hybrid electric vehicle(SHEV), a parallel hybrid electric vehicle (PHEV), and a parallel/serieshybrid electric vehicle (PSHEV).

FIG. 1 illustrates an exemplary “electric only” capable hybrid vehiclethat is commonly referred to as a parallel/series hybrid vehicle (PSHEV)system 10. The system 10 includes an engine 14, a transmission 16, and abattery 20 which operate with a planetary gear set 24, a generator 26, amotor 28, and meshing gears 32 to provide the torque. The torque isreceived by a torque shaft 36 fort transfer to a differential axle 38mechanism for final delivery to wheels 40.

The system 10 provides torque for driving the hybrid vehicle. The mannerin which torque is provided is variable and controllable by a vehiclesystem controller 44. FIG. 2 illustrates the variable and controllablemeans by which the vehicle system controller 44 can control powerdistribution in the system 10 for providing torque to the wheels 40.

In general, fuel is delivered to the engine such that the engine 14 canproduce and deliver torque to the planetary gear set 24. The powerprovided from the engine 14 is expressed as T_(e)ω_(e), where T_(e) isengine torque and ω_(e) is engine speed. Power delivered from theplanetary gear set 24 to the meshing gears 32 is expressed asT_(r)ω_(r), where T_(r) is ring gear torque and ω_(r) is ring gearspeed. Power out from the meshing gears 32 is expressed as T_(s)ω_(s),where T_(s) is the torque of shaft and ω_(s) is the speed of the torqueshaft, respectively.

The generator 26 can provide or receive power from the planetary gearset 24. This is shown with the double arrows and expressed asT_(g)ω_(g), wherein T_(g) is the generator torque and is ω_(g) thegenerator speed. As shown with path 48, the generator 26 can then supplypower to or receive power from the battery 20 or the motor 28 duringregenerative braking. As shown with path 50, the battery 20 can storeenergy received from the generator 26 and the motor 28 and it canrelease energy to the generator 26 and the motor 28. As shown with path52, the motor 28 provides power to and receives power from the generator26 and the battery 20. In addition, the motor 28 provides power to andreceives power from the meshing gears 32. This is shown with the doublearrows and expresses as T_(m) ω_(m), where T_(m) is motor torque andω_(m) is motor speed.

FIGS. 3–6 provide further illustration of the flow of power and theproduction of torque in the system 10.

FIG. 3 illustrates a positive split mode of operation. In this mode, theengine power is split between the meshing gears 32 and the generator 26,respectively. The splitting of power is controlled by the planetary gearset 24. The meshing gears 32 use the power received from the planetarygear set 24 to provide torque to the wheels 40. The battery 20 and themotor 28 can be controlled to receive power from generator 26. The motor28 can provide torque to the meshing gears 32 based on power receivedfrom one or both of the generator 26 and the battery 20.

FIG. 4 illustrates a negative split mode of operation. In this mode, thegenerator 26 inputs power to the planetary gear unit 24 to drive thevehicle while the motor 28 acts as a generator and the battery 20 ischarging. It is possible, however, that under some conditions the motor28 may distribute power to the meshing gearing 32, in which case thebattery 20 would power both the generator 26 and the motor 28.

FIG. 5 illustrates a parallel mode of operation. In this mode, agenerator brake 60 is activated and the battery powers the motor 28. Themotor 28 then powers the meshing gearing 32 simultaneously with deliveryof power from the engine 14 delivered to the meshing gearing 32 by wayof the planetary gear set 24. Alternatively, the motor 28 can act as agenerator to charge the battery 20 while the engine 14 provides power tothe wheels 40 or during regenerative braking.

FIG. 6 illustrates an electric only mode. In this mode, a one way clutch62 brakes the engine. The motor 28 draws power from the battery 20 andeffects propulsion independently of the engine 14, with either forwardor reverse motion. The generator 26 may draw power from the battery 20and drive against a reaction of the one-way coupling 62. The generator26 in this mode operates as a motor.

The vehicle system controller 44 (VSC) selects the power and torquedelivery mode based on the vehicle operating conditions and a predefinedstrategy. To this end, the vehicle system controller 44 receives asignal from a transmission range selector 66 (PRND), a desired enginetorque request 68, as shown at, which is dependent on accelerator pedalposition sensor output (APPS), and a brake pedal position sensor 70(BPPS).

In response to the received signals, the vehicle system controller 44generates signals to the engine 14, a transmission control module 74(TCM), and a battery control module 76 (BCM). Theses signals include adesired engine torque 80, a desired wheel torque 82, a desired enginespeed 84, a generator brake command 86, a signal 88 indicating batterycontractor or switch is closed after vehicle “key-on” startup. Themodules then provide further signal to control the hybrid vehicle, suchas a generator brake control 90, a generator control 92, and a motorcontrol 94.

The vehicle system controller 44 and the other control modules, includesensors and software algorithms that can be used to detect electrical,mechanical, software and other conditions in the system 10.

For the purposes of the present invention, a primary drivetraindesignation and an auxiliary drivetrain designation are provided. Thesedesignations are meant to cover all types of hybrid vehicles and todifferentiate between the drivetrains of the different hybrid vehicles.In particular, which are based on consumption based power sources, suchas an engine or a fuel cell, and storage based power sources, such abattery.

In detail, the primary drivetrain includes all the software,electronics, and mechanisms required for the engine 14, or fuel cell ifused, to provide torque to the wheels 40. The auxiliary drivetrainincludes all the software, electronics, and mechanisms required forproviding torque to the wheels when the engine is shut-off.

For the parallel/series hybrid vehicle shown in FIG. 1, the generator26, the battery 20, and the motor 28 are the primary components of theauxiliary drivetrain, in combination with the planetary gear set 24 ifneeded or available depending on the condition, selected gears of themeshing gears 32, and the torque shaft 36 used to transfer torque to thedifferential axle mechanism 38 for final delivery to wheels 40.

The vehicle system controller 44 monitors the primary drivetrain and theauxiliary drivetrain for an interruption or permanent disruption to thesoftware, electrical, or mechanical function of any item in thedrivetrains which would indicate future unavailability of the primarydrivetrain to produce energy for storage in the auxiliary drivetrain.

For the exemplary hybrid system shown in FIG. 1, unavailability of theprimary drivetrain would correspond to an condition which would renderthe engine 14 unsuitable for providing torque to the wheels 40 orunsuitable for providing power to the generator 26 or the battery 20 foruse by the motor 28 in providing torque to the wheels 40. In otherwords, unavailability of the primary drivetrain means the auxiliarydrivetrain must provide the torque to the wheels without anyreplenishment of power from the primary drivetrain, i.e. the engine or afuel cell.

When the engine 14, or a fuel cell if used, of the primary drivetrain isunable to provide torque to the wheels or replenish energy consumed bythe auxiliary drivetrain, the vehicle will gradually stop due to lack ofavailable power. Regenerative braking can occur in the auxiliarydrivetrain, but it will typically not be sufficient for prolongeddriving.

With respect to the exemplary system shown in FIG. 1, the operation ofthe auxiliary drivetrain is generally limited to the available batteryenergy remaining in the battery 20 during unavailability of the primarydrivetrain. This is due to the unavailability preventing the use of theengine 14 to replenish the energy in the auxiliary drivetrain, exceptfor possibly some limited replenishing by regenerative braking.

The limiting relates to limiting work done by the electric motor 28relative to its normal operating parameters. In other words, a drivingcharacteristic of the electric motor 28, such as power output andvehicle speed, is limited so that the HEV performance can be controlledto use less power, and in most cases decreased, to prolong operation ofthe HEV. The limited operation is commonly referred to as a limp homefeature.

The performance is limited according to performance limiting strategieswhich are based in part on the future availability or unavailability ofthe primary drivetrain to produce energy for storage in the auxiliarydrivetrain. The performance is limited further when the primarydrivetrain is unavailable in the future. This is done to prolongoperation of the vehicle by causing the electric motor to consume lessenergy than it would otherwise consume if the primary drivetrain wereavailable.

The severity of the limiting is based on the available battery energyremaining in the battery 20 after the condition and as its continuedconsumption. The vehicle system controller 44 can measure or calculatethe available battery energy to determine the limiting.

The vehicle system controller 44 can determine a battery voltage, abattery state of charge, or a battery discharge power limit to determinethe available battery energy and the corresponding limitation of thedriving characteristics, such speed and power.

One limitation technique relates to setting a maximum driving speed ofthe HEV. By controlling the maximum driving speed, the vehicle systemcontrol can insure the battery energy required to achieve relativelyhigh vehicle speeds is limit and used to prolong vehicle operation atlower speeds.

As shown in FIG. 7, the maximum vehicle speed can be controlled as afunction of the battery state of charge. Preferably, the maximum speedis set to 45 mph if the battery state of charge is above 50%, 35 mph ifthe battery state of charge is between 50% and 45%, 25 mph if thebattery state of charge is between 45% and 35%, and 15 mph if thebattery state of charge is between 35% and 20%.

Another limitation technique relates to setting a maximum power outputof the electric motor. By controlling the maximum power output, thevehicle system controller can control the rate of energy consumption. Inthis manner, the operation of the vehicle is less important that howrapidly the energy is being consumed. In other words, the vehicle speedand acceleration is indirectly controlled by setting the maximum poweroutput of the electric motor.

As shown in FIG. 8, the maximum power limit of the electric motor can becontrolled as a function of the battery state of charge. Preferably, themaximum power limit is set to 25 kW if the battery state of charge isabove 50%, 15 kW if the battery state of charge is between 50% and 45%,10 kW if the battery state of charge is between 45% and 25%, and 5 kW ifthe battery state of charge is between 35% and 20%.

Another limitation technique relates to setting a maximum speed andmaximum power limit. This combined control approach limits both thevehicle speed and the power expense of achieving the vehicle speed.

As shown in FIG. 9, the combined limits for setting the maximum vehiclespeed and the maximum power can be controlled as a function of thebattery state of charge. Preferably, the combined limits are set to 45mph and 25 kW if the battery state of charge is above 50%, 35 mph and 15kW if the battery state of charge is between 50% and 45%, 25 mph and 10kW if the battery state of charge is between 45% and 35%, and 15 mph and5 kW if the battery state of charge is between 35% and 20%.

Another limitation technique relates to a relationship for controllingactual power provided by the electric motor as a function of the maximumspeed and maximum power limits. In this manner, the consumption ofenergy from the battery is dynamically controlled based on real timemonitoring of HEV operation.

Preferably, the actual power is limited according to the followingalgorithm:

${AP} = {{2*{MP}*\left( \frac{{MS} - {VS}}{MS} \right)} - {{MP}*\left( \frac{{MS} - {VS}}{MS} \right)^{2}}}$wherein:

AP=actual power (kW);

MP=maximum power (kW);

MS=maximum vehicle speed (mph); and

VS=actual vehicle speed (mph).

FIG. 10 illustrates the effect of the actual power limit algorithm forselected maximum power and maximum speed limits. In particular, thevalues correspond with a curve L1 for a maximum speed of 45 mph and amaximum power of 25 kW if the battery state of charge is above 50%, witha curve L2 for a maximum speed of 35 mph and a maximum power of 15 kW ifthe battery state of charge is between 50% and 45%, a curve L3 for amaximum speed of 25 mph and a maximum power of 10 kW if the batterystate of charge is between 45% and 35%, and a curve L4 for a maximumspeed of 15 mph and a maximum power of 5 kW if the battery state ofcharge is between 35% and 20%.

The control of the actual power is based on dynamically controlling theactual power provided by the electric motor as a function of the currentvehicle speed. In this manner, an inverse relationship is set up betweenspeed and power such that less power is available as the vehicle speedapproaches the maximum speed limit and more power is available as thevehicle speed decreases relative to the maximum speed limit.

In addition to limiting a driving characteristic of the electric motorperformance, the limiting can comprise shutting down the vehicle if theavailable battery energy becomes so low that the primary drivetrain maynot be restarted. This may only be applicable to electric start hybridshaving engines which require starting torque from the energy storingdevice. Preferably, the vehicle is shutdown if the battery state ofcharge drops below 20%.

As described above, the various limiting techniques utilized batterystate of charge to indicate the available battery energy remainingduring the condition. Each of the limiting techniques could be executedbased on other energy indicators for the battery, such as voltage,discharge limit, or other substitute for battery state of charge.

The limiting focused on limiting a driving condition of the electricmotor by directly controlling the electric motor. Alternatively, theelectric motor could be passively controlled by, for example,controlling the battery such that the energy provided by the battery iscontrolled. By controlling the battery directly, the drivingcharacteristics of the electric motor can be passively limited by theoutputted battery power.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

1. A method for use in an electrically driven vehicle having a primarydrivetrain and an auxiliary drivetrain, the method comprising:controlling a driving characteristic of the auxiliary drive train basedon a first performance limiting strategy, the first performance limitingstrategy based in part on future availability of the primary drivetrain;and switching control of the driving characteristic to a secondperformance limiting strategy, the second performance limiting strategybased in part an future unavailability of the primary drivetrain;wherein the second performance limiting strategy comprises setting aplurality of maximum vehicle speed ranges based on battery state ofcharge for a battery used to power an electric motor of the auxiliarydrivetrain; and wherein setting the maximum vehicle speed rangescomprises setting the maximum vehicle speed to one of 45 mph if thebattery state of charge is above 50%, 35 mph if the battery state ofcharge is between 50% and 45%, 25 mph if the battery state of charge isbetween 45% and 35%, and 15 mph if the battery state of charge isbetween 35% and 20%.
 2. A method for use in an electrically drivenvehicle having a primary drivetrain and an auxiliary drivetrain, themethod comprising; controlling a driving characteristic of the auxiliarydrive train based on a first performance limiting strategy, the firstperformance limiting strategy based in part on future availability ofthe primary drivetrain; switching control of the driving characteristicto a second performance limiting strategy, the second performancelimiting strategy based in part on future unavailability of the primarydrivetrain and controlling power output of an electric motor of theauxiliary drivetrain; wherein the second performance limiting strategycomprises setting a plurality of maximum vehicle speed and a maximumpower ranges for the electric motor based on battery state of charge fora battery of the auxiliary drivetrain; and wherein setting the maximumspeed and the maximum power ranges comprises setting the maximum speedand the maximum power to one of 45 mph and 25 kW if the battery state ofcharge is above 50%, 35 mph and 15 kW if the battery state of charge isbetween 50% and 45%, 25 mph and 10 kW if the battery state of charge isbetween 45% and 35%, and 15 mph and 5 kW if the battery state of chargeis between 35% and 20%.
 3. A method for use in an electrically drivenvehicle having a primary drivetrain and an auxiliary drivetrain, themethod comprising; controlling a driving characteristic of the auxiliarydrive train based on a first performance limiting strategy, the firstperformance limiting strategy based in part on future availability ofthe primary drivetrain; switching control of the driving characteristicto a second performance limiting strategy, the second performancelimiting strategy based in part on future unavailability of the primarydrivetrain and controlling power output of an electric motor of theauxiliary drivetrain; wherein the second performance limiting strategycomprises limiting actual power provided by the electric motor to drivethe vehicle; and wherein the actual power is limited according to thefollowing algorithm${AP} = {{2*{MP}*\left( \frac{{MS} - {VS}}{MS} \right)} - {{MP}*\left( \frac{{MS} - {VS}}{MS} \right)^{2}}}$wherein: Al =actual power (kW); MP =maximum power (lcW); MS =maximumvehicle speed (mit); and VS =actual vehicle speed (mph).
 4. The methodof claim 3 further comprising setting maximum power and maximum speedranges based on the battery state of charge.
 5. The method of claim 4wherein setting the maximum speed and the maximum power ranges comprisessetting the maximum speed and the maximum power to one of 45 mph and 25kW if the battery state of charge is above 50%, 35 mph and 15 kW if thebattery state of charge is between 50% and 45%, 25 mph and 10 kW if thebattery state of charge is between 45% and 35%, and 15 mph and 5 kW ifthe battery state of charge is between 35% and 20%.
 6. A hybrid electricvehicle system having a primary drivetrain and an auxiliary drivetrain,the auxiliary drivetrain including a battery and an electric motor, thesystem comprising: a vehicle system controller for controllingconsumption of available battery energy by the electric motor, theenergy consumption controlled according to first and second performancelimiting strategies, the first performance limiting strategy based inpart on future availability of the primary drivetrain, the secondperformance strategy based in part on future unavailability of theprimary drivetrain and controlling power output of the electric motor ofthe auxiliary drivetrain; wherein the second performance limitingstrategy limits performance of the electric motor to consume less energythan the electric motor would consume for the first performance limitingstrategy; wherein the second performance limiting strategy compriseslimiting actual power provided by the electric motor to drive thevehicle; wherein the actual power is limited based on a relationshipbetween a maximum power limit and maximium speed limit for the electricmotor; and wherein the vehicle system controller sets maximum speed andmaximum power ranges to one of 45 mph and 25 kW if the battery state ofcharge is above 50%, 35 mph and 15 kW if the battery state of charge isbetween 50% and 45%, 25 mph and 10 kW if the battery state of charge isbetween 45% and 35%, and 15 mph and 5kW if the battery state of chargeis between 35% and 20%.
 7. A hybrid electric vehicle system, the systemcomprising: an internal combustion engine; a planetary gear setconnected to the internal combustion engine; a number of meshing gearsconnected to the planetary gear set to receive torque from the planetarygear set; a pair of wheels connected to the meshing gears to drive thevehicle; a generator connected to the planetary gear set; a batteryconnected to the generator for storing energy produced by the generator;an electric motor connected to the battery to consume energy from thebattery and to provide torque to the meshing gears; a vehicle systemcontroller for controlling consumption of available battery energy bythe electric motor during a condition which prevents the engine fromproviding torque to the wheels and power to the battery by controllingpower output of the electric motor based on the available batteryenergy; wherein the vehicle system controller determines the availablebattery energy by calculating a battery state of charge and limitsactual power provided by the electric motor to drive the vehicle; andwherein the actual power is limited according to the algorithm${AP} = {{2*{MP}*\left( \frac{{MS} - {VS}}{MS} \right)} - {{MP}*\left( \frac{{MS} - {VS}}{MS} \right)^{2}}}$wherein: AP =actual power (kW); MP =maximum power (lcWl; MS =maximumvehicle speed (mph; and VS =actual vehicle speed (mph).
 8. A method foruse in an electrically driven vehicle having a primary drivetrain and anauxiliary drivetrain, the method comprising: controlling a drivingcharacteristic of the auxiliary drive train based on a first performancelimiting strategy, the first performance limiting strategy based in parton future availability of the primary drivetrain; switching control ofthe driving characteristic to a second performance limiting strategy,the second performance limiting strategy based in part on futureunavailability of the primary drivetrain and controlling power output ofan electric motor of the auxiliary drivetrain; wherein the secondperformance limiting strategy comprises setting a plurality of maximumpower ranges for the electric motor based on battery state of charge fora battery of the auxiliary drivetrain; and wherein setting the maximumpower ranges comprises setting the maximum power to one of 25 kW if thebattery state of charge is above 50%, 15 kW if the battery state ofcharge is between 50% and 45%, 10 kW if the battery state of charge isbetween 45% and 25%, and 5 kW if the battery state of charge is between35% and 20%.